The primary cilium, a signaling antenna projecting at the surface of the cell, is required for the transduction of Hedgehog and planar cell polarity signals and concentrates many signaling receptors on its surface. Furthermore, ciliary dysfunction leads to obesity, retinal degeneration and kidney cysts in the inherited disorder Bardet-Biedl Syndrome (BBS). Yet, the mechanisms of signaling receptor trafficking to the ciliary membrane are not understood. We recently discovered a stable complex of seven BBS proteins, that we named the BBSome and have implicated in vesicular trafficking to the cilium based on its functional interaction with Rab8, a GTPase with a well-established trafficking function whose manipulations directly impact cilium growth. Here, we advance the hypothesis that the BBSome sorts specific transmembrane proteins to the primary cilium. In this research proposal, we will dissect BBSome function within the context of vesicular transport to the cilium through the following aims: 1- Identify the transmembrane proteins transported by the BBSome towards the cilium. Known ciliary transmembrane proteins will be tested for a BBSome requirement in their trafficking to cilia and we will establish a time-resolved trafficking assay to identify the donor compartment from which the BBSome selects its cargoes. 2- Dissect the mechanisms of BBSome targeting to cilia. We will assay the function of BBSome-binding proteins in mediating the recruitment of the BBSome to cilia. 3- Characterize the functional interplay between the BBSome and Rab8. In preliminary studies, we have found that the BBSome interacts with Rabin8, the GDP/GTP exchange factor for Rab. Here, we will seek to understand how the BBSome modulates the activity and localization of Rabin8 to enable Rab8 entry into the cilium. In conclusion, our model for BBSome function has significant implications for the etiology of Bardet-Biedl syndrome: in a model where the BBSome targets specific signaling receptors to the cilium, each individual symptom of BBS results from the disruption of a specific ciliary signaling pathway.